a) Field of the Invention
The invention is directed to an illumination device for microscopes which is suitable particularly for brightfield illumination.
b) Description of the Prior Art
The object of an illumination device for microscopes is to illuminate the visual field completely and uniformly. Since practically all of the light sources that are used generate an inhomogeneous light distribution in the visual field or in the planes conjugate to the visual field, corresponding steps must be taken to make the illumination homogenous. In microscopy, special halogen lamps are usually employed. These special halogen lamps have filament surfaces arranged vertical to the optical axis of the illumination device and have the largest possible illumination surface, although the latter is generally rectangular or square, that is, deviates from the circular shape of the visual field or conjugate fields. It is known to project the image of the filament surface directly next to or at a slight offset to the filament by means of a suitable reflector so as to generate an increase in the luminous area or a reduced filament structuring of the luminous area. However, the homogeneity of the illumination obtained in this way is often still not sufficient to meet the requirements for microscopy that are necessary in particular for applying Köhler illumination. Therefore, a large number of suggestions have been made for the elimination of this disadvantage. These suggestions generally consist in introducing special optical elements into the beam path, which special optical elements themselves do not have a homogeneously distributed light transparency. It is known from AT-PS 183 246 to arrange a matt surface with a circular non-matt surface portion in the beam path. DE 41 02 507 discloses a deflector disk having a plurality of individual prisms which are uniformly distributed over the entire surface and which image the light source in the aperture stop plane with a plurality of secondary images. EP 613 569 describes a raster disk with a plane-parallel outer area allowing an unobstructed passage of light and a circular inner area which is formed as a raster which images the light source multiple times. It is known from DE 198 32 665 to use a graduated gray filter having a radial grayscale graduation that is radially symmetric with respect to its center and in which the absorption coefficient of the filter decreases toward the edge. The light intensity which generally falls off toward the outer side according to the cosine law is compensated in this way. In order to realize this grayscale graduation, the text proposes arranging small opaque surface elements on a glass disk in such a way that the local surface ratio between the applied surface elements and the uncoated glass disk determines the transmittance of the filter. Since all of the proposed solutions still generate small residual inhomogeneities of the light intensity in the visual field, a scatter disk is usually used in addition to blur these inhomogeneities.
All of these known solutions have a number of disadvantages. For example, the production of these raster plates or filters is complicated and therefore expensive. The special halogen lamps with rectangular filament surfaces that are used are also relatively expensive. A considerable portion of the light energy is lost through the additional optical elements arranged in the beam path, which leads to the use of higher-output lamps which, in turn, give off increased heat and accordingly require additional steps for eliminating this heat.